There's so many old-school myths and past "truths" that are no longer true as we learn more, so I'd love to get a more up-to-date understanding of the differences between modern, well-designed and made 4:2:1/Tri-Y headers and 4:1 headers that use good merge angles, spears and properly designed merge collectors.

Discussions that pertain only to racing are still useful but I'm particularly interested in mild-to-hot Street applications.

The traditional rules-of-thumb / advice that 4:1 headers have huge advantages over 4:2:1 at high RPM but give up a lot in mid to low RPM torque and do nothing to prevent reversion as overlap increases & that 4:2:1/Tri-Y headers are for low to mid RPM torque improvement, reduce reversion and are largely appropriate for low RPM truck and street motors and fall on their face at high RPMs seem to be increasingly untrue as you look at the newer improvements to both designs. (Old street designs like the Doug Thorley 4:2:1 Tri-Ys come to mind immediately, advertising that they're better than 4:1s "up to 4,500 RPM"; note: I'm in no way bashing these, simply highlighting an example of a Tri-Y design clearly in the "improve low end torque at the expense of high end HP" camp. )

In the 4:2:1 space we now see high RPM race motors using Tri-Ys and folks like Calvin Elston highlighting that a properly designed 4:2:1 is better across all RPMs up to 8,500 RPM, so by making the primaries shorter (and all the other changes that comes with it), it seems that the typical weakness of the 4:2:1s, no longer has to be.

In the 4:1 space, merge collectors and more appropriately sized primaries and smaller collector chokes can dramatically increase cylinder scavenging and exhaust velocity and, I believe help with reversion some, too.

So how SHOULD we think about the differences between 4:1 headers and 4:2:1 headers and the situation that merits the selection of each one, today in 2017, assuming a modern design and particularly with a view towards max-effort street motors? Is the difference in the below 8,000 RPM space simply one of much smaller differences than before or one where as long as you have a well-designed header today either style header can be appropriately matched to any engine?

I've really tried to dive-in on this subject in the past week and I'm far more confused than when I started; I've got plenty of datapoints and opinions and cutting through the fog is now proving difficult.

I don't think that the tri-y design ever had a disadvantage. I think that the tri-y's being built for a long time were for applications that ran 5500 RPM or less, so people assumed they were not good for higher RPM engines. With the proper tube diameters and lengths, the tri-y design can be used for nearly any performance or racing engine. The NHRA Pro Stocks used tri-y's for a while, and they were finding power with the tri-y design. I'm glad to see that Elston is building tri-y's, and I hope to see more companies building them. Norm/CFS

So how SHOULD we think about the differences between 4:1 headers and 4:2:1 headers and the situation that merits the selection of each one, today in 2017, assuming a modern design and particularly with a view towards max-effort street motors?[/b] Is the difference in the below 8,000 RPM space simply one of much smaller differences than before or one where as long as you have a well-designed header today either style header can be appropriately matched to any engine?

Thanks in advance,
Adam

Once again, if you came to me with a "street" engine, it is cammed for power below 8000 rpm, I would do everything I can to fit a 421 header. It will always make more "power" under the curve, it will have more manifold vacumn at idle and be more responsive. All those are to me the most important part of a "street" engine, (and race engines also). If I hit the design well the first time, (experience) you would have a very hard and expensive time trying to find a 4into1 header that would make more power under the curve. And if you could compare headers, they both have to fit in the same car and use the same exhaust system.

The other side of this issue is if you have to buy a header in a box from your local speed shop? If it fits and doesn't burn things up or leak? Your good. Worry about something else.

I don't think that the tri-y design ever had a disadvantage. I think that the tri-y's being built for a long time were for applications that ran 5500 RPM or less, so people assumed they were not good for higher RPM engines.

The problem is that, at least in the off-the-shelf SBC market, the only available TRI-Ys are still those old designs. Some state, straight from the MFGR, that they're for "up to 4,500 RPM" and other "up to 5,500 RPM" (But then caveat that 5,500 RPM with a clarification that around 4,000 to 4,500, most 4:1s will surpass them in HP).

There's lots of shorter, more modern appearing TRI-Ys for the LS and Coyote markets, but nothing for SBCs. I'm trying to build a "2019 1979" Corvette with an SBC that takes advantage of as much modern tech as possible (roller everything, thin low pressure ring packages, sequential multiport fuel injection, crank trigger wheel, coil-near-plug LS ignition coils, etc...) -I've been pretty shocked to learn that, of all the things, it's the off-the-shelf exhaust systems that are keeping my engine stuck in 1979...

The Stan's Tri-Ys are so far rated for the highest RPMs of any tri-y's, I can find, but I'm quite certain that they're designed for a peak torque RPM, WELL below my engine's peak torque RPM of 4,500 RPM... Maybe that's a good thing for me as fuel economy is quite high on my list of priorities and I'm assuming these headers would pulls my torque RPM down and provide plenty of torque at highway cruise RPM with my 4L60e trans, but a secondary goal of my build is to show that a long-runner (19.25" runner start to valve) intake (First Fuel Injection-Monster-that-ate-the-TPI) CAN continue to make power to 6,000 RPM. The monster, big-block-like torque from a 3rd wave tuned intake (ala TPI) and high velocity heads & exhaust, but also with respectable HP @ 6,000, is for my goals anyway, better than an LS intake and available for my SBC. I loves me the long-runners. Finding an off-the-shelf exhaust that's matched to the build's reasonable street goals, is proving shockingly difficult. (Peak torque @ 4,500 and peak HP @ 6,500 for a 350; 6,000 for a 383--shouldn't be hard-to-find, right? -Seems like a reasonable street goal, no?

Why are there only 1970s header designs available off-the-shelf for SBCs? So weird...

So how SHOULD we think about the differences between 4:1 headers and 4:2:1 headers and the situation that merits the selection of each one, today in 2017, assuming a modern design and particularly with a view towards max-effort street motors?[/b] Is the difference in the below 8,000 RPM space simply one of much smaller differences than before or one where as long as you have a well-designed header today either style header can be appropriately matched to any engine?

Thanks in advance,
Adam

Once again, if you came to me with a "street" engine, it is cammed for power below 8000 rpm, I would do everything I can to fit a 421 header. It will always make more "power" under the curve, it will have more manifold vacumn at idle and be more responsive. All those are to me the most important part of a "street" engine, (and race engines also). If I hit the design well the first time, (experience) you would have a very hard and expensive time trying to find a 4into1 header that would make more power under the curve. And if you could compare headers, they both have to fit in the same car and use the same exhaust system.

The other side of this issue is if you have to buy a header in a box from your local speed shop? If it fits and doesn't burn things up or leak? Your good. Worry about something else.

Thanks again, Calvin! Definitely, helps me to come to the conclusion that Tri-Ys are the right choice for me. I wish there was an in-box header available that seemed to not prove a major restriction to producing power. (Off-the-shelf options are designed with HP peaks 1,500 - 2,000 RPM below my goal.) I'll probably grab a set of Stan's Tri-Ys and just be blissfully ignorant of all the HP left on the table from 5,000-6,500 RPM.

I just start considering the "hole" in my curve from 5,000-6,500 RPM with the off-the-shelf tri-ys so I wanted to pause to consider whether a good 4:1 merge collector-based header design could help keep the torque up in the lower RPM band and give me an over-all improvement in average power for the $. Unless there's a contingent who chimes in that they think a modern merge collector-based design is a better street option in terms of average power for the $$, I'm probably going to remain in the Tri-Y camp.

If you want to run triY's and you are not going for a custom set and you cant make up your mind then I would suggest you purchase pipemax, run your engine details into it and see what it suggests for your combo. You can then compare the pipemax info with the available shelf pipes and see if any are reasonably close or at least weed out any choices that have any lengths or diameters that are very wrong. You can then buy the closest shelf set and even if you had to modify a collector or secondary pipe length its not a big deal.
On the flip side, for what you are doing a shelf set with a GOOD exhaust system will do fine for the rpm range you want. One beauty of triY's is typically more engine bay room and less engine bay heat when compared to 4-1's.
Dont overthink it, you'll go mad ! Either commit to a big $ custom set, select a shelf set via pipemax or just buy something with primary pipe diameter that fits your ports and pop a good exhaust on them.

1. I'm going with Tri-ys (Calvin's advice is pretty unambiguous.)
2. I've saved a post previously where Larry Meaux talked about how to apply PipeMax recommendations to TRI-Y designs so I'll dig that up again
3. I'll either buy the off-the-shelf option or see what the folks at Stans would charge to make a custom tri-y based upon the PipeMax recommendations (I live in Seattle so Stans is pretty close to me.)

I recall reading about header development for a corvette engine a few years ago, and they did arrive at a tri-y design not far from what I'd expect. I'm sorry I don't recall the details, but I think you could find it with google. The engines you have mentioned are Corvette and an mx5, very common, I think other people have already done the work for you in this case, not that that's the end of the story but you could at least start where they left off.

1. I'm going with Tri-ys (Calvin's advice is pretty unambiguous.)
2. I've saved a post previously where Larry Meaux talked about how to apply PipeMax recommendations to TRI-Y designs so I'll dig that up again
3. I'll either buy the off-the-shelf option or see what the folks at Stans would charge to make a custom tri-y based upon the PipeMax recommendations (I live in Seattle so Stans is pretty close to me.)